Question:
A number is selected at random from the numbers 3, 5, 5, 7, 7, 7, 9, 9, 9, 9 The probability that the selected number is their average is
(a) $\frac{1}{10}$
(b) $\frac{3}{10}$
(c) $\frac{7}{10}$
(d) $\frac{9}{10}$
Solution:
GIVEN: A number is selected from the numbers 3,5,5,7,7,7,9,9,9,9
TO FIND: Probability that the selected number is the average of the numbers
Total numbers are 10
Average of numbers
$=\frac{3+5+5+7+7+7+9+9+9+9}{10}$
$=\frac{70}{10}$
$=7$
Total numbers of numbers which are average of these numbers are 3
We know that PROBABILITY = 
Hence Probability that the selected number is the average of the numbers is 
Hence the correct option is